Change ECS iteration

[englercj]

Currently iteration is:

• For each system, update all entities in system

This unfortunately means entities of some system may always render after another dur to system priority reguardless of the entity z index.

Because of that, I think entity should have some sorting property (zIndex, or renderPriority, or something) and the iteration should be:

• For each entity, apply each system to entity.

Obviously still only applying the systems that the entity is eligible for. One caveat being that for batch-based systems, this means we likely would need to bring back ObjectRenderer and the associated state :(